The present invention relates to a toner image fixing device that is used in electrophotographic copying machines, facsimiles, printers and the like machines using electrophotographic process.
In conventional electrophotographic copying machines, facsimiles, printers and the like machines using electrophotographic process, there is usually used such a toner image fixing device that fixes a toner image on a recording medium (hereinafter described as recording paper) passing through nip portion formed between a fixing roller and a pressure roller, by heating either one or both rollers (hereinafter called "pressure roller type").
In the pressure roller type fixing device, paired rollers must rotate in synchronism with each other and must be rotatably supported. Therefore, the device has a complicated construction and is large and expensive to manufacture.
To solve the above-mentioned problems, the present applicant proposed a toner image fixing method that fixes a toner image on recording paper transferring through nip portion formed between a fixing roller and non-rotating pressure member used instead of a pressure roller (Japanese Patent Application No.7-44647 filed on Mar. 3, 1995). This method is hereinafter called as "pressure-pad type".
A pressure-pad type toner image fixing device has a fixing roller which is a thin-wall aluminum cylinder whose external surface is covered all over with a coat of excellent toner-releasing, paper-guiding heat-resistant synthetic resin, e.g., heat-resistant silicone rubber having a large friction coefficient, a pressure member which is disposed under the fixing roller and a heat-resistant sheet on interposed between the pressure member and the fixing roller. The heat-resistant sheet is made of a 100 micron thick glass fiber base coated or impregnated with toner-releasing and heat-resistant synthetic resin, e.g., fluorocarbon resin: PFA (Tetra-fluoroethylene-fluoro-alkylvinylether copolymer), PTFE (Polytetrafluoro-ethylene) and FET (Polytetrafluoro-ethylene-Polytetra-fluoropropylene copolymer).
A recording paper with a not-yet-fixed toner image developed thereon passes nip portion formed between the fixing roller and the heat-resistant sheet for fixing the toner image thereon by fusing.
Problem To Be Solved
The pressure-pad type fixing device encounters a problem that the heat-resistant sheet causes a large amount of curling of a front edge of a recording sheet for the following reasons:
(1) In the conventional pressure roller type fixing device, the pressure roller has a large heat-capacity and can therefore heat the toner-image-carrying recording paper uniformly from the front edge to the tail end while the recording paper passes the fixing roller. The substantially even quality of the fixed toner image is obtained on the front end and the tail end of the recording paper. On the contrary, the pressure pad type fixing device uses a small heat-capacity fixing roller whose heat is transferred to a recording paper passing therethrough and which, therefore, can not evenly heat the recording paper. In other words, the tail portion of the recording paper is heated by a reduced heat from the fixing roller and therefore the toner image is is not applied uniformly to on the front portion and tail portion of the recording paper.
If the fixing temperature of the fixing roller is set at such a level that the toner image can be satisfactory fixed on the tail portion of the recording paper, the front end of the recording paper is excessively heated and curls up.
(2) In comparison with conventional pressure roller type method, the pressure pad type method provides wide nip portion for obtaining a high fixing quality of toner and therefore causes a large amount of curl of the front end of the recording paper.